I'd highly recommend getting it. DS2's story line is actually running concurrently with DS1. I found the beginning of DS1 a bit slow but once I got into the dungeons/puzzles, I started to really enjoy it.
Depends. DS2 explains the general story of begining and the very end of DS1, but not what the hell War was doing on earth, or the fact that he has been released which you only find out much later into the game in sort of cameo apearance. The thing is here, of just how much you want to play it and why, since you can find the story from the youtube. But if you want to enjoy both the story and fun gameplay then certainly buy DS1.